Retro-Diels-Alder-process

A special method, with only two examples, starts from 1,2,4-triazines.20 21 Diels-Alder reaction with the strained dienophile dimethyl tricyclo[4.2.2.02,5]deca-3,7,9-triene-7,8-dicarboxylate (14) is followed by an elimination of nitrogen via a retro-Diels-Alder process. The formed product, however, cannot be isolated, but reacts via another retro-Diels-Alder reaction and an electro-cyclic reaction to provide the azocine derivative 15. The sequence order of the reactions is not clear, but both pathways lead to the same product. 

Cycloaddition of 125 with buckminsterfullerene (Ceo) at 3 kbar allowed the adduct [48] to be obtained, preventing a retro Diels-Alder process (Scheme 5.19). Cycloadditions of tropone (125) with furans 134 gave mixtures of 1 1 endo-dcad exo-monocycloadducts 135 and 136, respectively [49a], together with some bisadducts. In this case furan reacts solely as the 27t component in spite of its diene system. Whereas 2-methoxy furan gave mainly the kinetically controlled product 135 (R= OMe Ri =R2 =H), under the same conditions 3,4-dimethoxy furan afforded the thermodynamically controlled cycloadduct 136 (R=H Ri =R2 =OMe) as the major product (Scheme 5.19). 

While most other techniques failed, MS was quite successful in distinguishing between the two isomers (20) and (21) (Scheme 5). While the 7-one (20) undergoes a retro Diels-Alder process and exhibits a diagnostic fragmentation pattern at m/z 203 and 102, these peaks are lacking in (21)... 

JHC297>. In a continuation of the above work, l,3,4-thiadiazolo[3,2-a][l,3,5]-triazines were also analyzed by electron impact mass spectrometry. A retro Diels-Alder process gave two fragments (Equation (1)) which in turn were further fragmented into smaller molecules. The mass spectral data for the 11 compounds analyzed has been tabulated in great detail <890MS(24)379>. 

Aqueous hetero Diels-Alder reaction was first described by Grieco, who reported the use of water as solvent for cyclocondensations of iminium salts (Larsen and Grieco, 1985). Known as being a very energy demanding reaction, the retro Diels-Alder process is usually not considered as a competitive pathway in most Diels-Alder... 

The dihydropyridazine (764) is unstable and loses nitrogen rapidly to yield a 1,3-diene (765) (69JA777, 72CC1260). Since (764) is generated by hydrolysis and oxidation of the cycloadduct (763) prepared from azodicarboxylate and2,4-hexadiene, it has proven possible to functionalize the double bond of (763) prior to nitrogen extrusion. Cyclopropanation, hydrolysis, decarboxylation and oxidation of (763) produce the 2,4-heptadiene (767) in a stereospecific manner via a concerted, orbital symmetry allowed retro-Diels-Alder process (Scheme 177). 

Several examples of allowed syn cycloadditions follow, but the reader may recall the 4 + 2 Diels-Alder and retro Diels-Alder processes in equations (20) and (21), parts c and d, and the 4 + 2 dipolar cycloaddition in (26). 

Mass spectral data have been reported for an oxetane derivative of (1) formed by the reaction of benzophenone with l-acetyl-pyrrolo[2,3-6]pyridine (Scheme 1). The major fragments upon electron impact result from a retro-Diels-Alder process to give the original component ions. These ions then undergo further fragmentation <76JHC585>. 

The reaction of oxazoles with alkynes is entirely different, leading to furans. The adducts (157) eliminate a cyanide in a retro-Diels-Alder process (equation 15). A typical example is the formation of the ester (164) from 5-ethoxy-4-methyloxazole and dimethyl acety-lenedicarboxylate (equation 16) equation (17) illustrates the production of two regioisomers in this reaction (79MI41802) a more elaborate case is the twofold addition of benzyne to 4-methyl-2,5-diphenyloxazole to give the bridged dihydroanthracene shown in equation (18) (80TL3627). 

Since perfluoroalkyl-substituted olefins and alkynes possess low-lying frontier orbitals, [4 + 2] cycloaddition reactions to oxazoles and thiazoles without strongly electron-donating substituents are unfavorable. On the other hand, five-membered heteroaromatic compounds possessing an electron-rich diene substructure, like furans, thiophenes, and pyrroles, should be able to add perfluoroalkyl-substituted olefins as well as alkynes in a normal Diels-Alder process. A reaction sequence consisting of a Diels-Alder reaction with perfluoroalkyl-substituted alkynes as dienophile, and a subsequent retro-Diels-Alder process of the cycloadduct initially formed, represents a preparatively valuable method for regioselective introduction of perfluoroalkyl groups into five-membered heteroaromatic systems. 

Certain quinazolines, e,g. l-methylquinazolin-4(l//)-one, fragment by a retro-Diels-Alder process. The MIKE (mass analyzed ion kinetic energy) spectrum of the molecular dication of quinazoline is very similar to that of quinoxaline. MIKE spectra of quinazolinethiones show the loss of NCS radicals from the molecular ionA ... 

One report has appeared of a system in which the first-formed cyclobutene adds a second molecule of alkyne photochemically, leading eventually to a benzene derivative as a result of electrocyclic ring-opening and a retro-Diels-Alder process (equation 55). 

In the case of the 1,4-anthracenophadiyne 18, TCNE was exclusively added to the 9 and 10 positions of the anthracene ring to give quantitative yield of an adduct (97) . When the adduct 97 was refluxed in tetrachloroethane, an isomeric one-to-one adduct (98) was obtained in 38% yield, probably via a retro Diels-Alder process... 

It is well known that Diels-Alder reaction of oxazoles as azadienes with acetylenic dienophiles result in the formation of furan derivatives via elimination of a nitrile from the adduct in a retro Diels-Alder process... 

The rates of loss of ethylene from 1,4- C2-labelled, 2,3- C2-labelled and various D-labelled tetralins following FI have been measured over the range from tens of picoseconds to microseconds [524, 770]. Interpretation of results was complicated by loss at short times of two other neutrals, viz. ethyl and ethane. Ethylene was lost by both a retro Diels—Alder process and another process effecting loss of C-1 and C-2 (or C-3 and C-4) [524]. Hydrogen exchanges between the benzenoid ring and the alicyclic ring did occur, but only after 10 ns [770]. 

Interestingly, isomer 2 is opposite to what one obtains from thebaine and glyoxylate esters (see Chapter 4). If isomer 2 is warmed, it isomerizes to 3 via a retro-Diels-Alder process. One major drawback of this methodology is that the starting sulfenyl chloride has a tendency to react directly... 

The spectral features of 1,4-disubstituted quinolizidines are quite similar to those of the 5,8-disubstituted indolizidine analogues. Their mass spectra are characterised by a major fragment arising from a-cleavage of the alkyl side-chain at C-4 and by an additional peak at m/z 110 (C H 2N+, ca. 30-50%) in ion-trap spectra, arising from a subsequent retro Diels-Alder process, Fig. (5). Since, as seen for the indolizidine analogues, the alkyl substituent at C-l is commonly a methyl group the base peak of... 

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